1. Field of the Invention
This invention relates to a photodiode for an LD/PD module for bidirectional optical communication system with signal light of two wavelengths xcex1 and xcex2 via a single optical fiber which can suppress the influence of transmitting light upon the photodiode. The single-fiber bidirectional optical communication system using of a single fiber both for transmitting and receiving signals employs an LD/PD module. The LD/PD module has a substrate, a package, an LD emitting transmission signals and a PD receiving signals mounted upon the common substrate in the common package. This is called a multiwavelength bidirectional LD/PD module.
Another optical communication system transmits more than one signal by the light of plural wavelengths in one direction via a single fiber and receives the multiwavelength signals by a PD module having a plurality of photodiodes. The PD module has more than one photodiode on a common substrate in a common package. This is a unidirectional multiwavelength PD module.
A photodiode is a sensor which converts light power (light signals) to a photocurrent (electric signals) in proportion to the light power. The PD is sometimes called an O/E transducer or O/E sensor. The photodiode is a highly sensitive sensor. The LD generates strong light power for transmitting optical signals to a far distanced port. Although the wavelengths are different for transmitting signals and for receiving signals, the photodiode which has sensitivity also to the transmission wavelength has a possibility of sensing the transmission power yielded by the laser diode mounted on the same package.
This application claims the priority of Japanese Patent Applications No.11-201519 (201519/1999) filed on Jul. 15, 1999 and No.11-260016 (260016/1999) filed on Sep. 14, 1999 which are incorporated herein by reference.
The phenomenon that the PD senses the transmission signal emitted from the LD which is stored in the same package is called xe2x80x9coptical crosstalkxe2x80x9d. The LD light of the same port is noise for the PD. Sensing the transmission light at the same port hinders the PD receiving the transmission light from the counterpart port (e.g., central station). It is important to suppress the crosstalk from the LD (transmitting device) to the PD (receiving device) at the same package. There are two interactions between the transmitting device and the receiving device on the same substrate. One is the optical crosstalk which is optical coupling between the PD and the LD. The other is the electric crosstalk which is conveyed from the LD to the PD by electromagnetic waves. Both kinds of crosstalk are difficult for the LD/PD module to conquer. This invention aims at solving the optical crosstalk.
There are several versions of the bidirectional LD/PD module for making use of a single fiber both for transmission and reception with regard to the modes of signal separation of the transmitting light and the receiving light. A typical signal separation device is a WDM (wavelength division multiplexer) which divides the common path spatially to a transmission path and a reception path by the difference of the light wavelengths. The WDM separation alleviates the difficulty of the optical crosstalk, since the WDM allots different paths to the LD and the PD for separating them spatially. A special disposition is a serial alignment of the PD and the LD on the same straight line. In this case, most of the transmission path is common with the reception path. The common path type module is suffering from more serious crosstalk problem.
This application claims the priority of Japanese Patent Applications No.11-201519(201519/19990) filed on Jul. 15, 1999 and No.11-260016(260016/1999) filed on Sep. 14, 1999, which are incorporated herein by reference.
2. Description of Related Art
FIG. 1 shows a typical multiwavelength bidirectional optical communication system having LD/PD modules at a central station and at a subscriber port. At the station, an LD1 generates downward signals. The downward signals travel through an optical fiber 1, a WDM 2, an optical fiber 3, another WDM 4 and an optical fiber 5 to a PD2 at the subscriber site. The PD2 converts the downward optical signals to electric (received) signals. At the ONU (optical network unit) terminal, a subscriber LD2 generates upward optical signals. The upward signals spread through an optical fiber 6, the WDM 4, the fiber 3, the WDM 2 and the fiber 7 and attain at a PD1 at the station. The PD2 converts the light signal from the subscriber into electric signal. The single fiber 3 enables both the upward signals and the downward signals to spread in both directions between the central station and the ONU terminal. The WDM 2 at the station alternatively allocates the downward signals and the upward signals into the fiber 1 or fiber 7 by the difference of the wavelengths. The downward light wavelength is denoted by xcex2. The upward light wavelength is designated by xcex1. Both signals are propagating in both directions in the same fiber 3. The WDM 4 at the subscriber port (ONU) alternatively allocates the downward signals and the upward signals into the fiber 5 or fiber 6 by the difference of the wavelengths. The ONU receives the downward (receiving) signals xcex2 by the PD2. The LD2 generates the transmission (upward) signals xcex1 at the ONU. Electric circuits following the PD2 and the LD2 are omitted in FIG. 1.
The words of xe2x80x9ctransmittingxe2x80x9d or xe2x80x9creceivingxe2x80x9d signals have reverse directions (or inverse flows) at the station and at the ONU. In the description, the words should be defined at the ONU site. Thus, the upward light xcex1 corresponds to the transmitting signals. The downward light xcex2 carries the receiving signals. The prior art (multiwavelength bidirectional communication) of FIG. 1 separates the PD2 and the LD2 spatially by dividing the light paths by the WDM.
FIG. 2 shows a prior multiwavelength unidirectional optical communication system for transmitting various signals from a central station in a downward direction to a subscriber port. At the station, an LD1 and an LD2 generate different downward signals of xcex1 and xcex2. The downward signals travel through an optical fiber 1 or 7, a WDM 8, an optical fiber 3, another WDM 4 and an optical fiber 5 or 6 to a PD1 or a PD2 at the subscriber site. The WDM 4 separates two different signals by the difference of the wavelengths xcex1 and xcex2. The PD1 senses xcex1. The PD2 detects xcex2. Crosstalk occurs between PD1 and PD2 also in the multiwavelength unidirectional system.
FIG. 3 is a sectional view of a prior art PD module which has widely been used as a receiving device in the optical communication network having the spatially separated paths as shown in FIG. 1 or FIG. 2. The PD module has a metallic bottom circular stem 10 with lead pins 9 extending downward. A PD chip 12 is mounted via a submount 11 onto the stem 10. A thin metal cap 14 having a lens 13 is adjusted and welded on the stem 10. A cylindrical sleeve 15 is adjusted and welded on the stem 10 above the cap 14. A ferrule 16 is inserted, adjusted and fixed in an axial hole of the sleeve 15. The ferrule 16 clamps an end of an optical fiber 17. The end of the ferrule 16 is slantingly polished. An elastic bend-limiter 18 caps the top end of the sleeve 15 for protecting the fiber 17 from overbending. FIG. 1 and FIG. 2 include LD modules in addition to the PD modules explained by FIG. 3. The LD module is omitted to describe, since it is simply obtained by replacing the PD chip with an LD chip in the module of FIG. 3.
This invention is applicable to the spatially separating LD/PD or PD/PD modules as shown in FIG. 1 or FIG. 2. The prior PD module of FIG. 3 has a three-dimensional structure making use of a metallic package. The expensive metal package hermetically seals the PD device and shields the PD from external noise. Although the metal-can PD module has advantages of the sealing property and the shielding property, such a PD module has drawbacks of the number of parts, the necessity of adjustment, the number of steps of fabrication and the high cost. High cost prevents the metal-can type PD, LD or LD/PD modules from pervading widely.
Flat planar type LD, PD or LD/PD modules have been intensively investigated as low-cost devices. The flat planar type is called xe2x80x9cPLC (planar lightguide circuit)xe2x80x9d-type. FIG. 4 denotes an example of a PLC type PD module containing a bottom-incidence type PD. This invention can also be applied to the PLC module, which is now explained preliminarily. The PLC module is constructed upon a rectangular silicon bench (Si-bench) 19. The Si-bench 19 has a V-groove 20 formed by selectively etching the Si substrate from an end to a midway point in an axial direction in the middle. A slanting mirror plane 21 is formed by the same etching at the final end of the V-groove 20. A PD chip 23 is mounted just above the slanting mirror plane 21 upon the Si-bench 19. The PD chip 23 is a bottom surface incidence type PD with a light sensing part 24 at the top. An optical fiber 22 is fitted into and fixed to the V-groove 20. The light beam emanating from the fiber 22 travels in the V-groove 20 and shoots the mirror 21. Being reflected upward by the mirror 21 the signal light goes into the PD 23 via the bottom surface and attains to the sensing part 24. A photocurrent flows in proportion to the power of the signal light. The PLC type module succeeds in excluding the adjustment operation. Exclusion of adjustment alleviates the cost by facilitating production.
Both the PD modules of FIG. 3 and FIG. 4 can be applied for detecting the receiving signals separated by the WDMs in FIG. 1 and FIG. 2. The WDM is made, for example, by making a light waveguide branch having wavelength selectivity in a lightguide layer on a Si substrate. WDMs have various types with different shapes, different materials or different principles. FIG. 5 shows a prism type WDM. The WDM has two transparent glass columns 25 and 26 with a rectangular triangle section. The glass columns have a slanting surface coated with a dielectric multilayer 27. The dielectric multilayer 27 has wavelength sensitivity which allows one wavelength to pass through but reflects the other wavelength. The sensitivity enables the WDM to integrate the receiving light and the transmitting light. The reciprocal property of reflection or refraction laws allows the same WDM to act both as a wavelength integrating device and a wavelength separating device. In FIG. 5, the xcex2 receiving light going out of a fiber 28 is reflected by the dielectric layer 27 and reaches a PD 30. The transmitting light xcex1 made by an LD 29 goes through the dielectric layer 27 and enters the fiber 28. This invention can contribute to such an LD/PD module including a PD and an LD spatially separated by the WDM.
The WDM separating type LD/PD modules, however, are less significant than unseparating path type LD/PD modules for the present invention. The unseparating path type LD/PD modules mean the module which has a PD without an inherent path separated from the LD path. The unseparating type modules have been first proposed by the present Inventors. The unseparated path type LD/PD module aligns a PD and an LD on an axial line which is an extension of the optical fiber. The PD has no exclusive path different from the path of the LD. Alignment of the PD and the LD on the same axial line simplifies the structure of the module. This is a strong point. The same path for the LD and the PD1 however, enhances the difficulty of the crosstalk more seriously than the WDM separating type LD/PD module. FIG. 6 denotes an example of the unseparated path type LD/PD which has bigger significance to the present invention. A direct purpose is to solve the problem of the crosstalk in the unseparated path type LD/PD modules of FIG. 6.
In the figure, a dotted rectangle denotes a housing (package) 31. The housing 31 has a Si-bench which is omitted here for simplicity. An optical fiber 32 is mounted in an axial direction in the housing 31. An LD 33 is installed at an extension of the fiber 32 in the housing 31. A WDM filter 35 slantingly cuts the fiber 32 at a spot distanced from the LD 33. The WDM filter 35 discriminates the light by the wavelength. A PD 34 is mounted above the WDM filter 35. The inherent path from the WDM 35 to the PD 34 is quite short. The LD 33 yields strong transmission light xcex1 of about 1 mW or more. The LD-emitted strong light travels to the left in the fiber, passes the WDM 35 and makes a long journey in the fiber 32 to e.g., the station. The receiving light xcex2 propagating to the right in the fiber, is reflected by the WDM 35 to the PD 34. A receiving region 36 of the PD 34 detects the receiving light xcex2. The transmitting light xcex1 is strong light. The receiving xcex2 is weak light. xcex1 and xcex2 have a common path from the beginning end to the WDM. The propagating directions are reciprocal for xcex1 and xcex2 and the WDM 35 separates xcex1 and xcex2. Sometimes the strong LD light partially invades into the PD 34 due to the short distance between the common axial line and the PD. The invasion of the LD light causes the optical crosstalk. Even a small rate of the LD light raises a large noise in the PD, since the LD power is strong enough and the receiving light is weak.
If a conventional PD as shown in FIG. 7 were used as the PD in FIG. 6, the invasion of the strong LD power would cause more than noise due to the wide range of sensitivity. The LD-noise would prevail over the receiving signal in the PD. The S/N rate (signal/noise rate) would be far smaller than 1. The conventional InP type PD chip is fabricated from an epitaxial wafer having an n-InP substrate 37 and epitaxial layers consisting of an n-InP buffer layer 38, an n-InGaAs light receiving layer 39 and an n-InP window layer 40 epitaxially deposited in turn on the n-InP substrate 37. A p-region 41 is formed by diffusing zinc (Zn) from the center of the n-InP window layer 40. An annular p-electrode 42 is formed on the p-region 41. An antireflection film 43 is made on the p-region 41 within the annular p-electrode 42. A passivation film 44 is deposited on the InP window layer 40 out of the p-electrode 42 for protecting the pn-junction. An n-electrode 45 is deposited on the bottom of the InP substrate 37. In the case of the upper surface incidence type as shown in FIG. 7, the bottom n-electrode 45 is an overall coating metal film and the top p-electrode 42 is annular. In the case of the bottom incidence type, the bottom n-electrode should be annular and the top p-electrode should be an overall metal film.
FIG. 8 shows the sensitivity distribution of the prior InGaAs-type photodiode (PD) of FIG. 7 A front falling part (P) at shorter wavelengths corresponds to the band gap of the InP window layer 40. The light of a shorter wavelength than P is insensitive to the PD, because the shorter wavelength light is absorbed by the InP window layer 40. Another falling part (R) at longer wavelengths corresponds to the band gap of the InGaAs light receiving layer 39. The light having longer wavelength of lower energy (hv) is insensitive to the PD, because the longer wavelength light is not absorbed by the InGaAs light receiving layer 39. Thus, the conventional InGaAs-type PD has a wide range (Q) of sensitivity from the band gap (P) of the InP window layer 40 to the band gap(copyright) of the InGaAs light receiving layer 39. The sensitivity range (Q) includes both 1.3 xcexcm and 1.55 xcexcm bands. The prior PD has sufficient sensitivity both for the 1.3 xcexcm and 1.55 xcexcm bands.
The prior PD of FIG. 7 is generally adopted for sensing the infrared light in the optical communication. The PD has a wide sensitivity range from 1 xcexcm to 1.65 xcexcm as shown in FIG. 8. The wide sensitivity range is a strong point of the PD because of the probable common use of the PD both for receiving the 1.3 xcexcm and 1.55 xcexcm light. However, when the wide sensitive PD is used in the LD/PD module, the PD is apt to cause large crosstalk owing to the wide sensitivity range including the 1.3 xcexcm band.
If the transmitting light xcex1 had lower energy than the receiving light xcex2 (xcex1 greater than xcex2), a contrivance of the light receiving layer would enable the PD to decrease the crosstalk. The PD can sense the light having higher energy than the band gap energy of the light receiving layer by absorbing the light at the light receiving layer and converting the light energy into photocurrent. But the PD is insensitive for the light having energy lower than the band gap of the light receiving layer, since the light passes through the light receiving layer without loss. If the light receiving layer were made of a material having a band gap of an intermediate energy between the transmission light energy and the receiving light energy, the PD would sense only the receiving light exclusively.
If the transmitting light xcex1 had higher energy than the receiving light xcex2 (xcex1 less than xcex2), any contrivance of the light receiving layer would fail to decrease the crosstalk. The crosstalk cannot be suppressed by changing the band gap of the light receiving layer at all. Such case is just an object of the present invention. Namely, the transmission wavelength xcex1 is shorter than the receiving wavelength xcex2 in the present invention (xcex1 less than xcex2). For example, the transmission light is xcex1=1.3 82 m and the receiving light is xcex2=1.55 xcexcm. Poor performance of the WDM will induce large crosstalk in the WDM-separated type of FIG. 1 or FIG. 2. The unseparated path type LD/PD module incurs far greater crosstalk due to the short distance between the PD and the axial common signal line.
The reason why the LD/PD module of FIG. 6 is liable to invite large optical crosstalk will be described here. In FIG. 6, all the strong transmitting light emitted from the LD does not go into the optical fiber 32. A part of the laser (LD) light shoots the platform or the resin outside of the fiber due to the wide aperture of the LD light. The extra light is reflected, refracted and scattered by the platform (Si-bench) or the resin. Unfortunately, the silicon (Si-) bench is transparent to the LD transmitting light xcex1. The resin enclosing the LD is also transparent to the LD infrared light. This is an origin of the difficulty of the optical crosstalk. The Si bench and the resin reflect, refract and scatter the stray LD light. Scattering direction or scattering power depends upon the distribution of the resin, the shape of the platform or the disposition of the LD or PD. The loci of the scattering, refracting or reflecting light is complex. Whole of the platform seems to glow brightly to the PD. The random LD light out of the fiber is called xe2x80x9cstrayxe2x80x9d light or xe2x80x9cscatteredxe2x80x9d light.
The LD light is invisible to human eyes but sensible to the PD. The stray light xcex1 goes into the PD in various directions at various heights as inner noise. The noise LD light invades into the PD via the upper surface, the bottom surface and the side surfaces. Then, the stray LD transmission light causes the crosstalk without entering the optical fiber. Since the stray light does not pass the WDM filter, the stray light cannot be suppressed by an improvement of the performance of the WDM at all. Furthermore, the LD light once going into the fiber sometimes induces the crosstalk due to the leak from the fiber owing to the scattering. An increase of the LD power makes it difficult to suppress the leak of the LD xcex1 light. The NVDM is indifferent to the stray LD light.
Nobody had been aware of the fact. The skilled has believed that the imperfection of the WDM filter would invite the LD light to invading to the PD. They thought that the requisite should be a contrivance of eliminating the LD light in the short path from the WDM to the PD sensing region. The description of the present invention discriminates two modes of the noise LD light by calling the LD light not entering the fiber xe2x80x9cstray lightxe2x80x9d and the LD light once guided into the fiber but refracted by the WDM to the PD xe2x80x9cleak lightxe2x80x9d.
The Inventors had once contrived a superb PD having a selective absorption layer which does not absorb the xcex2 receiving light but absorbs the LD xcex1 light in the epitaxial layers. {circle around (1)} Japanese Patent Laying Open No.11-83619 (83619/1999) (Applicant: Sumitomo Electric Industries, LTD., Inventors: Yoshiki Kuhara, Hiromi Nakanish Hitoshi Terauchi, filled on Sep. 3, 1997) proposed such a PD. FIG. 9 shows the structure of the PD chip proposed by {circle around (1)}. An n-type InGaAs light receiving layer 47 is grown on an n-type InP substrate 46. A p-type region 48 is made at a central part of the n-InGaAs light receiving layer 47 by diffusion zinc (Zn) which is a p-dopant. A pn-junction denoted by a dotted curved line is formed as an interface between the n-type region and the p-type region. An i-layer (depletion layer) 49 accompanies the pn-junction just below. The p-region 48 has a p-electrode 50 on the top surface. A passivation film 51 (for example, SiN) is made on the n-InGaAs light receiving layer 47 out of the p-electrode 50 for protecting the edge of the pn-junction. An n-type InGaAsP absorption layer 52 is formed on the bottom of the n-InP substrate 46. The InGaAsP absorption layer 52 was the novel point of {circle around (1)}. An annular n-electrode 53 having an opening is formed upon the bottom of the absorption layer 52. The central opening of the n-electrode 53 is an entrance aperture of the PD chip. The entrance aperture is protected by an antireflection window 54 coating the absorption layer 52.
FIG. 10 denotes more a detailed structure of the PD chip suggested by {circle around (1)}. In practice, an n-InP buffer layer 56 is inserted between the n-InP substrate 46 and the n-InGaAs light receiving layer 47 for improving the crystallographic property of the light receiving layer 47. An n-InP window layer 55 is formed upon the InGaAs light receiving layer 47. Zn is thermally diffused from the InP window layer 55 to the InGaAs receiving layer 47 for making the p-region 48. If the PD lacked the InGaAsP absorption layer 52, the PD would have a wide sensitivity range (Q) as explained by referring to FIG. 8 due to the InP substrate 46 and the InP buffer layer 56 which give a tolerant lower limit (P).
The example is a bottom-incidence type PD, for example, applicable to the module of FIG. 6. The novel point is the InGaAsP absorption layer 52 in {circle around (1)}. Quaternary compound InGaAsP allows free choice of the band gap and the lattice constant. Semiconductors or insulators generally can absorb the light of energy higher than the band gap but cannot absorb the light of energy lower than the band gap. Wavelengths are inversely proportional to energy. A band gap wavelength xcexg is defined as the wavelength of the light having the energy equal to the band gap. Lower energy light (xcex greater than xcexg) passes the material but higher energy light (xcex less than xcexg) is partially or entirely absorbed by the material. Then, if the band gap of the quaternary InGaAsP is chosen as an intermediate value between the transmitting wavelength xcex1 and the receiving wavelength xcex2, the InGaAsP would not absorb the xcex2 but would selectively absorb the xcex1 light. This is the basic idea of {circle around (1)}.
For example, if the transmitting light wavelength is xcex1=1.3 xcexcm and the receiving light wavelength is xcex2=1.55 xcexcm, the band gap of the absorption layer should be about xcexg=1.40 xcexcmxcx9c1.46 xcexcm. The absorption layer has, e.g., carrier (electron) concentration of 1 xc3x971018 cmxe2x88x923 and a thickness of about 5 xcexcm. This is a considerably thick layer. The carrier concentration is also high. The large thickness (e.g., 5 xcexcm) is required for absorbing all the 1.3 xcexcm light without the rest. The InGaAsP of xcexg=1.40 xcexcmxcx9c1.461 xcexcm has attenuation coefficient xcex1=104 cmxe2x88x921 for 1.3 xcexcm light. A 5 xcexcm-thick layer gives attenuation of exp(xe2x88x92xcex1d) =0.007 which means very small transparency of 0.7%. The high electron concentration (1018 cmxe2x88x923) aims partially at prohibiting the forward resistance from rising due to the absorption layer and partially at facilitating the recombination of the electron-hole pairs yielded by the light. The absorption layer is effective to suppress the optical crosstalk between the PD and the LD, since the absorption layer does not absorb the receiving light xcex2 (1.55 xcexcm) but selectively absorbs the LD transmission light (1.55 xcexcm).
FIG. 11 shows the wavelength dependence of the transparency of the InGaAsP absorption layer 52 explained in {circle around (1)}. The compound ratio is selected for giving a band gap wavelength e.g., xcexg=1.42 xcexcm (xcexg=1.40 xcexcmxcx9c1.46 xcexcm). The absorption layer absorbs the light of a shorter wavelength than 1.42 xcexcm but allows the light of a longer wavelength than 1.42 xcexcm to pass. The sensitivity is 1A/W for 1.55 xcexcm but less than 0.01 A/W for 1.3 xcexcm. The extinction ratio is less than {fraction (1/100)} (20 dB).
Since the PD is the bottom incidence type, a mixture of the xcex1 and xcex2 light goes into the PD from the bottom aperture. The xcex1 light is fully absorbed by the absorption layer 52. No xcex1 light reaches the light receiving region (the depletion layer). The PD senses no xcex1 light. The WDM-scattered LD light causes no crosstalk in the improved PD. The PD is, in particular, effective to the unseparated path type LD/PD module as shown in FIG. 6. The PD is also effective to the separated path type LD/PD module of FIG. 1 or FIG. 2, since the absorption layer can annihilate the extra xcex1 light which has not been eliminated by the WDM filter. {circle around (1)} is a superb, excellent invention which provides the n-InP substrate 46 with the n-InGaAs light receiving layer 47 on the upper surface and the n-InGaAsP absorption layer 52 on the bottom surface. Epitaxy makes both the InGaAs light receiving layer 47 and the InGaAsP absorption layer 52. Both surface film formation makes the epitaxy difficult. The allocation of the absorption layer and the light receiving layer on both surfaces brings about an advantage annihilating electron-hole pairs borne by the absorption of xcex1 in the absorption layer 52 without making an influence upon the light receiving layer 47. The design of the thickness of the absorption layer required to consider the attenuation factor exp(xe2x88x92xcex1d) for the 1.3 xcexcm light. It was supposed that the absorption layer would annihilate the obstacle xcex1 light which escapes from the WDM filter to a enough small value of exp(xe2x88x92xcex1d). The absorption layer was thought to remove the crosstalk between the transmission signals and the receiving signals completely.
The assumption turned out not to be entirely true. {circle around (1)} is effective indeed to annihilate the xcex1 (1.3 xcexcm) leak which goes via the bottom into the PD, because the light passes the absorption layer. The WDM passing leak light is not the whole of the light emanating from the LD and going into the PD. Besides the WDM passing leak, some part of the LD light attains to the PD after being scattered or reflected at the fiber end, the resin or the substrate several times. This xcex1 light is called stray light for discerning it from the WDM-passing leak light. The LD chip has a short resonator length which is equal to the length of the PD chip. The short resonator admits the LD emitting light to spread in a wide aperture both vertically and horizontally because of using no lens. Considerable part of the LD light does not go into the fiber. Fiber-excluded, scattered LD light illuminates the whole of the Si bench brilliantly, in particular, in the unseparating type LD/PD module aligning the PD and the LD on an extension of the fiber axial line as shown in FIG. 6. This fact is beyond the imagination of the skilled. Since the Si bench is transparent to the xcex1 light, the xcex1 light propagates in the Si bench and the resin. The Si bench and the resin glitter with xcex1. Some part of the scattered xcex1 attains to the PD without passing the WDM. Enhancement of the WDM performance has no effect on preventing the fiber-excluded xcex1 from shooting the PD. It is important to remove the fiber-excluded xcex1 scattered (stray) light by the bench, the resin or the package as well as to eliminate the leak xcex1 light due to the imperfection of the WDM.
As defined before, the xcex1 light emanated from the LD but excluded from the fiber is called stray light. The stray xcex1 fills the housing as a whole. Although the PD is the bottom-incidence type, the xcex1 light stray does not necessarily come into the PD via the bottom. The PD has a big thickness of about 200 xcexcm. Some xcex1 stray light obliquely enters the PD via the sides. Some scattered stray light shoots the top of the PD. Other stray light goes into the PD via the bottom. Various paths guide the stray xcex1 light to the PD. The stray xcex1 light invading from the sides or the top makes its own way to the depletion layer 49 in the InGaAs receiving layer without touching the absorption layer 52. The stray light generates photocurrent in the depletion layer 49 or the p-region 48. This is noise current for the PD. The noise-generation is called crosstalk. Namely, the bottom absorption layer is impotent to prevent the stray light from entering the PD via the sides or the top. Furthermore it turns out that the side-top entering stray light is rather more powerful than the bottom entering stray light.
For example, it would be possible to widen the p-electrode 50 (in FIG. 9) on the light receiving layer and cover the other part of the receiving layer with an opaque film for suppressing the top entering stray light. It is far difficult to suppress the side-entering stray light. PD chips are made by repeating epitaxy, diffusion, lithography or other wafer processes on a circular wafer and by scribing the wafer into plenty of chips along the cleavage lined lengthwise and crosswise. It is impossible to treat with the sides of the chips any more. The height of the sides is about 200 xcexcm. The wide sides are not protected with any opaque films. The sides are exposed to a jam of the stray light.
The wavelength selectivity of WDM filters or WDM couplers is 15 dB to 20 dB at present. The receiving port requires at least 30 dB or desirably 40 dB of the wavelength selectivity as a whole. For example, if the LD power is 1 mW (=0 dBm), the least receivable power is xe2x88x9230 dBm and the S/N ratio of the noise (xcex1) to the signal to the PD is xe2x88x9210 dB, the required whole wavelength selectivity is 40 dB (30 dB+10 dB). Further, if the least receivable (detectable) power is xe2x88x9235 dB and the S/N ratio to the PD is xe2x88x9215 dB (about a thirtieth), the required wavelength selectivity is 50 dB (35 dB+15 dB).
A purpose of the present invention is to provide a PD structure which enables the PD to prevent both the stray LD light and the leak LD light from invading the PD in the LD/PD module having an LD yielding transmitting light of a shorter wavelength and the PD sensing receiving light of a longer wavelength.
To solve the problem, the present invention proposes two kinds of improved PDs. One PD (1) of the present invention has an absorption layer just below the light receiving layer but above the substrate. Namely, the absorption layer is sandwiched between the substrate and the light receiving layer. The absorption layer has the wavelength selectivity of allowing xcex2 to pass but annihilating xcex1. This is called a xe2x80x9csingle absorptionxe2x80x9d type. The other PD (2) of the present invention has an absorption layer just below the light receiving layer but above the substrate and another absorption layer below the substrate. Namely, the first absorption layer is sandwiched between the substrate and the light receiving layer. The second absorption layer covers the bottom of the substrate. Two absorption layers sandwich the substrate. This is called a xe2x80x9cdouble absorptionxe2x80x9d type.
(1) single absorption type
The PD has a single absorption layer below the light receiving layer and above the substrate. The previous PD {circle around (1)} has an absorption layer below the substrate as shown in FIG. 9 or FIG. 10. Unlike {circle around (1)}, this improved PD (1) has an absorption layer just above the substrate but beneath the light receiving layer. The absorption layer is thus sandwiched between the substrate and the light receiving layer. The absorption layer of the improved PD (1) is nearer to the light receiving layer than {circle around (1)}. In addition to the bottom-entering xcex1 light, the present invention absorption layer can annihilate the side-entering xcex1 light or the slantingly-entering xcex1 light. The noise xcex1 light does not attain to the sensing region (depletion layer or pn-junction), which decreases the crosstalk from the LD to the PD.
(2) double absorption type
The PD has a first absorption layer below the substrate and a second absorption layer below the light receiving layer and above the substrate. Unlike {circle around (1)} (FIG. 9 or FIG. 10), this improved PD (2) has two absorption layers. The first absorption layer covering the bottom of the substrate aims at annihilating the leak xcex1 light which comes into the PD via the WDM and the PD bottom. The first absorption layer compensates the imperfection of the WDM by eliminating the leak. The second absorption layer above the substrate below the light receiving layer has the role of eliminating both the xcex1 stray light and the xcex1 leak light. The second absorption layer can absorb widely the side-horizontally or side-slantingly entering xcex1 stray light besides the bottom entering light. The allotment of the roles is;
First absorption layer (bottom surface of substrate)=removal of leak light.
Second absorption layer (top surface of substrate)=removal of stray and leak light.
The xcex1 LD light which is noise to the PD is doubly absorbed by the first absorption layer and the second absorption layer. The xcex1 does not reach the sensing region of the PD. The PD does not sense xcex1, which depresses the optical crosstalk from the LD to the PD.
The present invention determined the compound ratio of the quaternary mixture InGaAsP of the absorption layer for providing the band gap wavelength xcexg with an intermediate value between xcex1 and xcex2 (xcex1 less than xcexg less than xcex2). If xcex1=1.3 xcexcm and xcex2=1.55 xcexcm, 1.3 xcexcm less than xcexg less than 1.55 xcexcm.
The thickness of the absorption layer relates to the annihilation rate of xcex1. The thickness d should range from 3 xcexcm to 10 xcexcm. About 5 xcexcm is a preferable thickness. Too thin an absorption layer cannot fully kills the xcex1 light. Too thick an absorption layer raises the cost through an increment of the material cost and the long layer growing time. Besides, a more than 5 xcexcm thick InGaAsP layer would be subject to degradation of the crystal structure. From the viewpoint of crystallography, the absorption layer should not surpass 5 xcexcm in thickness. The thickness will be considered later more in detail.
Another property for defining the absorption layer is carrier concentration. The carrier (electron) concentration should be rather high. A preferable carrier concentration is about 10 18 cmxe2x88x923. Too low carrier concentration would bring about the inconvenience of enhancing the forward resistance of the PD and taking a long time for recombining the electron-hole pairs excited by the xcex1 LD light in the absorption layer. Unlike the previous {circle around (1)}), extinction of holes by the recombination with electrons is significant in the present invention, which will be later explained further.
The present structure has wide applicability. The present invention can be applied to various types of PDs, for example, the bottom-incidence type PD, the top-incidence type PD, the side-incidence type PD and the waveguide type PD.
There is an additional contrivance for killing the LD light further. The side-entering xcex1 is fully perished by building a peripheral p-region (diffusion shield layer) around the central p-region on the PD. An extra depletion layer below the peripheral p-region absorbs light of all wavelengths entering via the sides and makes electron-hole pairs which will die away in the peripheral p-region without arriving at the central depletion layer.
A further improvement is to make an InP window layer on the InGaAs light receiving layer. The window layer suppresses dark current and ensures long-term reliability. A passivation film protects the pn-junction appearing to the surface.
Insertion of a low-doped InP buffer layer between the absorption layer and the light receiving layer is effective to improving the crystal structure of the light receiving layer. High dopant concentration is apt to disturb the lattice structure. High dopant concentration often perturbs the crystal structure of the absorption layer. Here, the low-doped buffer layer contributes to recovering the lattice structure of the absorption layer.
The thickness d of the absorption layer is explained here in detail. xcex1 is the absorption coefficient, d is the thickness and T is the transparency of the absorption layer. Transparency is defined as a rate of the outgoing power to the incidence power. Assuming that no reflection occurs at the top surface and the bottom surface, the transparency T is related to d and xcex1 by the equation,
T=exp(xe2x88x92xcex1d)xe2x80x83xe2x80x83(1)
xcex1 for xcex1 depends upon the compound ratio of InGaAsP. In the case of absorption edge wavelength of e.g., 1.42 xcexcm (xcexg=1.40 xcexcmxcx9c1.461 xcexcm), the absorption coefficient is xcex1=1xc3x97104 cmxe2x88x921 for 1.3 xcexcm. FIG. 22 denotes the relation between the thickness d and the transparency T. xcex1 depends on the wavelength (xcex1=xcex1(xcex)). The absorption layer does not absorb 1.55 xcexcm light (T=1) (xcex1(1.55 xcexcm)=0). T for 1.3 xcexcm denotes the filtering effect (extinction ratio) of 1.3 xcexcm to 1.55 xcexcm.
filtering effect=xe2x88x9210 log T=4.343 xcex1d.xe2x80x83xe2x80x83(2)
If xcex1=1xc3x97104 cmxe2x88x921, a 10 dB filtering effect (T=10%) requires a thickness more than d=2.3 xcexcm from FIG. 22. A 20 dB filtering effect (T=1%) demands d=4.6 xcexcm of the absorption layer. Due to the fluctuation of the thickness of epitaxial layers, the 20 dB filtering effect (T=1%) requires the absorption layer of a thickness d=4 xcexcm to 6 xcexcm. Namely, 5 xcexcm is a desirable thickness for the absorption layer.
A thicker absorption layer may be more desirable for increasing the filtering effect. However, a thicker quaternary compound crystal layer degrades the crystal property of the absorption layer. From the standpoint of the crystal property, 5 xcexcm is the preferable thickness. However, the InP buffer layer which improves the crystal property allows an about 10 xcexcm thickness for the absorption layer.
Discussion over T is sufficient for suppressing the LD leak via the WDM. This invention aims at annihilating not only the leak light but also the stray light unlike prior art {circle around (1)}. One problem is fast recombination of carriers. The noise xcex1 (e.g.,1.3 xcexcm) being absorbed by the absorption layer makes pairs of electrons and holes there. If the holes (minority carriers) crossed over the absorption layer to the above light receiving layer and recombined with electrons in the receiving layer, a photocurrent due to the noise xcex1 would flow in the PD and would blur the signal current. The holes (minority carrier) yielded in the absorption layer by the xcex1 light should be recombined with electrons (majority carriers) within the absorption layer. The absorption layer should have an enough thickness for allowing the holes to recombine with electrons and for restraining the holes from leaking into the light receiving layer. The absorption layer should be larger than the product of the hole lifetime and the hole velocity. Here, the absorption layer is an n-InGaAsP crystal. The majority carrier is an electron and the minority carrier is a hole. In the layer, electrons are majority carriers and the lifetime of the electron is insignificant.
Holes are minority carriers in the n-type absorption layer. Since the carrier (electron) concentration is large, there is no electric field (E=0) in the absorption layer like a metal. There is no electrostatic field there. Holes are not pulled by electric field in the absorption layer. However, there is hole density gradient in the layer. The hole density gradient pushes holes toward the region of lower hole density. The diffusion guides the holes toward the p-region. During the diffusion, the holes collide with majority electrons and recombine with the electrons. The recombination annihilates the holes in the absorption layer. The xcex1 yielded holes vanish in the absorption layer without inducing a photocurrent.
The diffusion distance from generation to extinction is called a diffusion length Ld. Hole diffusion distance is denoted by Ld h and electron diffusion distance is designated by L d e. The hole diffusion distance Ld h is defined as a root of a product Dxcfx84 of the diffusion coefficient D and the hole lifetime xcfx84. Ld h=(D Txc2xd. The diffusion coefficient D is defined as a limit of x2/t at txe2x86x920, where x is the displacement of the hole and t is the time of the diffusion. D=lim(x2/t).
In general, the electron diffusion distance Ld e is long but the hole diffusion distance Ldxe2x80x2h is short in the InGaAsP crystal. If a p-type absorption layer were employed for absorbing the noise xcex1 light, the minority carriers (electrons) would have a long diffusion distance which would cause a real photocurrent in the PD and would lower the S/N rate. It is inconvenient to employ the long lifetime carrier as the minority carrier. Thus, the preferable minority carrier is a hole having a short lifetime and the favorable conduction type of the absorption layer is n-type which has holes as minority carriers.
The diffusion length depends upon the compound ratio of InGaAsP of course. The diffusion length also depends on the purity of the crystal. Purer crystal causes less times of collision of holes. High doped crystal shortens the lifetime of holes due to the frequent collision. In a high pure InGaAsP crystal of a carrier density n=1015 cmxe2x88x923, the electron diffusion length is Ld e=6.0 xcexcm and the hole diffusion length is Ld h=1.6 xcexcm. An crystal of a lower purity shortens the diffusion lengths Ld e and Ld h owing to an increment of a recombination section (collision probability) (decreasing xcfx84) through an increase of the majority carriers.
The hole diffusion length Ld h decreases in proportion to a root of the carrier (electron) concentration n in the n-type region. The absorption layer is made of highly doped InGaAsP crystal (nearly n=1018 cmxe2x88x923). The high dopant density (n=1018 cmxe2x88x923) lowers the diffusion length to about a thirtieth (1/30) of the value in the high purity crystal (n=1015 cmxe2x88x923). The hole diffusion length is estimated to be Ld h=0.05 xcexcm in the absorption layer (n=10 18 cmxe2x88x923). Ld h=0.05 xcexcm means that the hole density reduces to 1/e in a 0.05 xcexcm distance.
Absorption does not mean an immediate extinction of the LD light power. At the instance of absorption, the light is converted into pairs of electrons and holes. xcex1 is a measure of the probability of the conversion. The InGaAsP layer having a large xcex1 converts almost all of the light into electron-hole pairs at a beginning part of the absorption layer. The diffusion of holes starts at an early part of the absorption layer. A 5 xcexcm thick absorption layer attenuates the light power down to exp(xe2x88x925/0.05)≈10xe2x88x9244. The high carrier concentration n of the absorption layer aims at shortening the hole diffusion length Ld h (denoted simply by xe2x80x9cLxe2x80x9d hereinafter). The lower limit of the carrier concentration n in the absorption layer depends upon the thickness d of the absorption layer. A bigger thickness d permits a smaller concentration n by admitting a longer diffusion length L.
Strictly speaking, the function of annihilating xcex1 should be considered by taking account of two different phenomena. One is a optoelectronic conversion from light to electron-hole pairs at a point distanced from the bottom of the absorption layer by z. The other is recombination of holes with electrons for extinction. The probability of the optoelectronic conversion is xcex1exp(xe2x88x92xcex1z)dz where dz is an infinitesimal thickness of the layer. A decreasing ratio of a hole borne at z against the recombination is given by exp{(z-d)/L} at the final point z=d of the layer, where L is the diffusion length, d is the thickness of the layer and z is an arbitrary point (0xe2x89xa6zxe2x89xa6d) in the layer. The surviving hole ratio S is given by an integration of the product of xcex1exp(xe2x88x92xcex1z) and exp{(z-d)/L} by z.                                                         S              =                              α                ⁢                                  ∫                                                            exp                      ⁡                                              (                                                                              -                            α                                                    ⁢                                                      xe2x80x83                                                    ⁢                          z                                                )                                                              ⁢                    exp                    ⁢                                          {                                                                        (                                                      z                            -                            d                                                    )                                                /                        L                                            }                                        ⁢                                          ⅆ                      z                                                                                                                                              =                              α                ⁢                                  xe2x80x83                                ⁢                                                      L                    ⁡                                          (                                              1                        -                                                  α                          ⁢                                                      xe2x80x83                                                    ⁢                          L                                                                    )                                                                            -                    1                                                  ⁢                                  {                                                            exp                      ⁡                                              (                                                                              -                            α                                                    ⁢                                                      xe2x80x83                                                    ⁢                          d                                                )                                                              -                                          exp                      ⁡                                              (                                                                              -                            d                                                    /                          L                                                )                                                                              }                                                                                        (        3        )            
xcex1 has the same physical dimension as 1/L. xcex1 has little dependence upon the carrier concentration of the absorption layer. The hole diffusion length L deeply depends upon the concentration n. The pure InGaAsP of n=1015 cmxe2x88x923 has a 1.6 xcexcm hole diffusion length. The diffusion length is in proportion to the root of the concentration n. For a general n, the hole diffusion length L is
L=1.6xc3x97(1015/n)xc2xd(xcexcm)xe2x80x83xe2x80x83(4)
Substitution of (4) to (3) yields a general expression of S for arbitrary d and n,                               10          ⁢                      xe2x80x83                    ⁢          log          ⁢                      xe2x80x83                    ⁢          S                =                  10          ⁢                      xe2x80x83                    ⁢                      log            ⁡                          [                              1.6                ⁢                                  xe2x80x83                                ⁢                                                      α                    ⁡                                          (                                                                        10                          15                                                n                                            )                                                                            1                    2                                                  ⁢                                                      {                                          1                      -                                              α                                                  1.6                          ⁢                                                                                    (                                                                                                10                                  15                                                                n                                                            )                                                                                      1                              2                                                                                                                                            }                                    ⁡                                      [                                                                  exp                        ⁡                                                  (                                                                                    -                              α                                                        ⁢                                                          xe2x80x83                                                        ⁢                            d                                                    )                                                                    -                                              exp                        ⁢                                                  {                                                                                    -                                                              xe2x80x83                                                            ⁢                                                              d                                1.6                                                                                      ⁢                                                                                          (                                                                  n                                                                      10                                    15                                                                                                  )                                                                                            1                                2                                                                                                              }                                                                                      ]                                                              ]                                                          (        5        )            
This is the extinction rate of the receiving light to the transmission light. For example, more than 20 dB of extinction ratio is required (10logS less than xe2x88x9220 dB). Eq.(5) teaches us the required carrier concentration n and the thickness d of the InGaAsP layer. For instance, d=5 xcexcm and n=1018 cmxe2x88x923 give S=xe2x88x9235 dB. And d=5 xcexcm and n=1017 cmxe2x88x923 give S=xe2x88x9230 dB. A low concentration n=1017 cmxe2x88x923 is also permitted. This is a lower limit of the concentration. The upper limit of n is determined by the restriction of preventing the crystal property from degrading. The upper limit of n is n=1019 cmxe2x88x923.
1017 cmxe2x88x923xe2x89xa6nxe2x89xa61019 cmxe2x88x923xe2x80x83xe2x80x83(6)
The differences between the prior art {circle around (1)} and the present invention (A,B) are now explained. The prior art {circle around (1)} has an absorption layer on the bottom of the substrate. One (A) of the present invention has an absorption layer on the top surface of the substrate next to the light receiving layer. The other (B) of the present invention has two absorption layers sandwiching the substrate.
{circle around (1)}; a single absorption layer below the substrate
A; a single absorption layer above the substrate below the light receiving layer
B; two absorption layers; one below the substrate, the other above the substrate,
In short, the present invention has an advantage of the wider solid angle xcexa9 than {circle around (1)}, which is an aperture from the receiving region to the absorption layer. This invention is superior to {circle around (1)} in the width of the solid angel xcexa9. The breadth of the PD chip is denoted by W (e.g., 300 xcexcm to 500 xcexcm). The vertical distance between the light receiving layer and the absorption layer is denoted by g. The solid angle xcexa9 which is an aperture looking from a point in the receiving (sensing) region at the absorption layer is given by
xcexa9=2xcfx80[1xe2x88x92g/{(W/2)2+g2}xc2xd].xe2x80x83xe2x80x83(7)
In the present invention (A, B), the distance g between the absorption layer and the sensing region is short enough, e.g., about 2 xcexcm to 10 xcexcm but the half breadth W/2 is large, e.g., about 200 xcexcm. Thus, this invention gives about xcexa9=2xcfx80 which is half of the whole solid angle (4xcfx80).
On the contrary, the distance g including the substrate is about 200 xcexcm to 300 xcexcm in the prior art {circle around (1)}. The solid angle of {circle around (1)} is about xcexa9=xcfx80. Thus the aperture (solid angle xcexa9) of the present invention is twice as wide as {circle around (1)}. The wider aperture is more effective to protect the PD from the stray light. The protection by the present absorption layer is more effective than {circle around (1)} by a factor of 2. The present invention can be discriminated from the prior art {circle around (1)} by the simple geometrical advantage.
First, the first invention A is compared with the prior art {circle around (1)}. There is a technical advantage on fabrication of the present invention A over the prior art {circle around (1)}. The prior art {circle around (1)} requires twice epitaxial growths for making an epitaxial light receiving layer on the top surface and another epitaxial absorption layer on the bottom surface. {circle around (1)} requires two steps of epitaxial growth on both surfaces of the substrate. The double epitaxial growth enhances the cost of epitaxy for {circle around (1)}. This invention A can make the absorption layer and the light receiving layer by a single epitaxial growth on the top surface of the substrate. Single surface epitaxy alleviates the step of epitaxy, which lowers the fabrication cost for the present invention A.
Second, the second invention B is compared with the prior art {circle around (1)}. The invention B has two absorption layers; the first absorption layer is below the substrate and the second absorption layer is above the substrate. Almost all of the WDM leak light from the bottom surface is eliminated by the first absorption layer. Even if some of the leak light passes the first absorption layer, the second absorption layer annihilates the leak light completely. The second invention B has double protection of the PD from the stray and the leak LD light. The invention B is far superior to the prior art {circle around (1)} in the removal of the optical crosstalk.
This invention proposes a new PD without crosstalk for the LD/PD module of the multiwavelength bidirectional or unidirectional optical communication system. The PD (A) of Invention A has a single absorption layer between the substrate and the light receiving layer. The PD (B) of Invention B has two absorption layers; a first absorption layer is piled on the bottom of the substrate, a second absorption layer is sandwiched between the substrate and the light receiving layer. The LD emits strong transmission light xcex1. A part of the xcex1 is scattered at random. The scattered xcex1 light (stray) fills the package. The stray light xcex1 shoots the PD in all directions. Imperfection of the WDM allows the xcex1 to shoot the aperture of the PD. The absorption layers prevent the stray xcex1 and the leak xcex1 light from coming into the light receiving layer. In the double absorption layer PD(B), the second absorption layer is effective to get rid of the xcex1 stray light and both the first and second absorption layers are effective to eliminate the xcex1 leak light.
From the standpoint of suppressing the leak light xcex1, a single 10 xcexcm thick absorption layer seems to be equivalent to an assembly of a first 5 xcexcm thick absorption layer and a second 5 xcexcm thick absorption layer. But it is not true. The absorption layer is a highly doped and quaternary mixture crystal. Too thick absorption layer degenerates the crystal property of the absorption layer. 10 xcexcm is the upper limit which can maintain the good crystallographical property for the quaternary, high-doped absorption layer. It is better to make two 5 xcexcm thick good absorption layers than to make a single 10 xcexcm thick absorption layer from the viewpoints of both the crystal property and the extinction of noise xcex1 light.
The role of the second absorption layer is not rigorously equal to the role of the first absorption layer even for the leak xcex1 light deriving from the imperfection of the WDM. The holes borne in the first absorption layer are all annihilated, since the thick substrate separates the first absorption layer from the light receiving layer. The holes made in the second absorption layer are not fully eliminated. Some of the holes can attain to the light receiving layer and yield photocurrent, since the second absorption layer is in contact with the light receiving layer. The first absorption layer is superior to the second absorption layer in annihilating the leak light xcex1. The assembly of a 5 xcexcm absorption layer and a 5 xcexcm absorption layer of the present invention is more effective than a single 10 xcexcm absorption layer.
This invention succeeds in reducing the optical crosstalk from the LD to the PD by suppressing the LD light xcex1 from coming into the light receiving layer. The PD of the invention is, in particular, effective to be applicable to the path unseparated type LD/PD module of FIG. 6. Of course, the PD of the present invention can be applied to the path-separated type LD/PD module shown in FIG. 1, FIG. 2 and FIG. 5.